The penis (fig. 226) is the male organ of copulation.

In a quiescent condition it is hidden away in a double fold of skin termed the "sheath" or "prepuce", from which, in a state of erection, it protrudes for some considerable distance. The posterior part of the organ between the perineum and the scrotum, being somewhat closely bound down to surrounding structures, is called the "fixed portion. The rest, which is enclosed in the sheath, and capable of being projected or withdrawn, is the "free" portion. It commences behind in two branches or crurse, which become attached to the ischial tuberosities, and after uniting pass forward between the thighs, where it is suspended by a layer of elastic tissue, and terminates in front in a rounded extremity enclosed by the glans penis.

The Penis.

Fig. 226. - The Penis.

1, Transverse Section of Penis. 1, Anterior Dorsal Arteries; a, Fibrous Capsule; B, Corpus Cavernosum; c, E, Accelerator Urinae Muscle; D, Urethra or urinary passage; F, Retractor Penis Muscle; G, Corpus Spongiosum; H, Septum Pectiniforme. 2, Longitudinal Section of Glans Penis. J, Glans Penis divided; K, Urethral Canal. 3, Penis unsheathed. L, Prepuce or Sheath; M, N, Mucous Membrane thrown into folds; O, Glans Penis.

When in a state of erection it is seen to be covered by a thin moist unctuous-looking membrane, which is a continuation of the loose skin forming the inner fold of the sheath. At the extremity of the organ this membrane is much reduced in thickness, and after closely investing the "glans" enters the orifice of the urethra, and becomes continuous with its mucous lining membrane.

For the most part the penis is made up of three elongated parallel bodies, the " corpora cavernosa " and the " corpus spongiosum ".

The two "corpora cavernosa" (b, fig. 226) commence behind in two tapering processes called the crurae, which, after uniting together at the root of the penis, continue forward to form the body, and terminate in two rounded extremities over which the glans penis fits like a cap.

When divided transversely, the corpora cavernosa are seen to be surrounded by a dense fibrous coat of considerable thickness. Although closely united together at their circumference, they are partly separated by an imperfect septum or partition, which, when viewed laterally, somewhat resembles the teeth of a comb; hence it has been termed the "septum pectiniforme"'. The partition is derived from the outer fibrous coat, and becomes more and more complete as it reaches the root of the penis. The interior of the corpora cavernosa is divided into a number of small cells by a net-work of elastic septa, which interlace each other in all directions, and give the divided surface the appearance of a sponge. At the time of erection all the spaces so formed are filled with blood, their elastic walls are put on the stretch, and the volume of the organ is correspondingly increased.

When the blood leaves the cells erection ceases.

The arteries of the corpora cavernosa are derived from the pudic.